Variable-nozzle assembly for a turbocharger

ABSTRACT

A variable-nozzle turbocharger includes a turbine housing and a center housing, and a generally annular nozzle ring and an array of vanes rotatably mounted to the nozzle ring such that the vanes are rotatably adjustable for regulating exhaust gas flow to the turbine wheel. An elastically deformable member is disposed between a radially inwardly facing surface of the nozzle ring and a radially outwardly facing surface of the center housing, the elastically deformable member having a radially inner surface contacting the radially outwardly facing surface of the center housing and having a radially outer surface contacting the radially inwardly facing surface of the nozzle ring so as to radially center the nozzle ring relative to the center housing.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/534,348 filed on Sep. 22, 2006, which is now U.S. Pat. No. 7,559,199,the entire disclosure of which is hereby incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to turbochargers having a variable-nozzleturbine in which an array of movable vanes is disposed in the nozzle ofthe turbine for regulating exhaust gas flow into the turbine.

An exhaust gas-driven turbocharger is a device used in conjunction withan internal combustion engine for increasing the power output of theengine by compressing the air that is delivered to the air intake of theengine to be mixed with fuel and burned in the engine. A turbochargercomprises a compressor wheel mounted on one end of a shaft in acompressor housing and a turbine wheel mounted on the other end of theshaft in a turbine housing. Typically the turbine housing is formedseparately from the compressor housing, and there is yet another centerhousing connected between the turbine and compressor housings forcontaining bearings for the shaft. The turbine housing defines agenerally annular chamber that surrounds the turbine wheel and thatreceives exhaust gas from an engine. The turbine assembly includes anozzle that leads from the chamber into the turbine wheel. The exhaustgas flows from the chamber through the nozzle to the turbine wheel andthe turbine wheel is driven by the exhaust gas. The turbine thusextracts power from the exhaust gas and drives the compressor. Thecompressor receives ambient air through an inlet of the compressorhousing and the air is compressed by the compressor wheel and is thendischarged from the housing to the engine air intake.

One of the challenges in boosting engine performance with a turbochargeris achieving a desired amount of engine power output throughout theentire operating range of the engine. It has been found that thisobjective is often not readily attainable with a fixed-geometryturbocharger, and hence variable-geometry turbochargers have beendeveloped with the objective of providing a greater degree of controlover the amount of boost provided by the turbocharger. One type ofvariable-geometry turbocharger is the variable-nozzle turbocharger(VNT), which includes an array of variable vanes in the turbine nozzle.The vanes are pivotally mounted in the nozzle and are connected to amechanism that enables the setting angles of the vanes to be varied.Changing the setting angles of the vanes has the effect of changing theeffective flow area in the turbine nozzle, and thus the flow of exhaustgas to the turbine wheel can be regulated by controlling the vanepositions. In this manner, the power output of the turbine can beregulated, which allows engine power output to be controlled to agreater extent than is generally possible with a fixed-geometryturbocharger.

The variable vane mechanism is relatively complicated and thus presentsa challenge in terms of assembly of the turbocharger. Furthermore, themechanism is located between the turbine housing, which gets quite hotbecause of its exposure to exhaust gases, and the center housing, whichis at a much lower temperature than the turbine housing. Accordingly,the variable vane mechanism is subject to thermal stresses because ofthis temperature gradient.

The assignee of the present application has previously addressed theissues noted above by providing a variable-nozzle turbocharger thatincludes a cartridge containing the variable vane mechanism, asdescribed in international patent application PCT/US2005/037622 assignedto the assignee of the present application. The turbine defines a nozzlethrough which exhaust gas is delivered to the turbine wheel, and acentral bore through which exhaust gas is discharged after it passesthrough the turbine wheel. The cartridge is connected between the centerhousing and the turbine housing and comprises an assembly of a generallyannular nozzle ring and an array of vanes circumferentially spaced aboutthe nozzle ring and rotatably mounted to the nozzle ring and connectedto a rotatable actuator ring such that rotation of the actuator ringrotates the vanes for regulating exhaust gas flow to the turbine wheel.The cartridge also includes an insert having a tubular portion sealinglyreceived into the bore of the turbine housing and having a nozzleportion extending generally radially out from one end of the tubularportion, the nozzle portion being axially spaced from the nozzle ringsuch that the vanes extend between the nozzle ring and the nozzleportion. A plurality of spacers are connected between the nozzle portionof the insert and the nozzle ring for securing the nozzle ring to theinsert and maintaining an axial spacing between the nozzle portion ofthe insert and the nozzle ring. The cartridge further comprises agenerally annular retainer ring fastened to the center housing in such amanner as to capture the nozzle ring between the retainer ring and thecenter housing, the retainer ring being formed as a separate part fromthe insert and being mechanically and thermally decoupled from theinsert.

The cartridge described in the aforementioned PCT application iseffective for providing stress decoupling of the variable vanemechanism. However, it is desired to reduce the overall size of theturbocharger while retaining the stress decoupling advantages of thecartridge design.

BRIEF SUMMARY OF THE INVENTION

The present invention addresses the above needs and achieves otheradvantages, by providing a variable-nozzle cartridge for a turbochargercontaining the variable vane mechanism. The turbine defines a nozzlethrough which exhaust gas is delivered to the turbine wheel, and acentral bore through which exhaust gas is discharged after it passesthrough the turbine wheel. The cartridge is connected between the centerhousing and the turbine housing and comprises an assembly of:

-   -   a generally annular nozzle ring and an array of vanes        circumferentially spaced about the nozzle ring and rotatably        mounted to the nozzle ring and connected to a rotatable actuator        ring such that rotation of the actuator ring rotates the vanes        for regulating exhaust gas flow to the turbine wheel, wherein        the nozzle ring includes a radially outer surface facing a        radially inner surface of the turbine housing, and wherein a        radial gap is defined between the radially outer surface of the        nozzle ring and the radially inner surface of the turbine        housing, the radial gap allowing radial displacement of the        radially outer surface of the nozzle ring relative to the        turbine housing;    -   an insert having a tubular portion sealingly received in the        bore of the turbine housing and having a nozzle portion        extending generally radially out from one end of the tubular        portion, the nozzle portion being axially spaced from the nozzle        ring such that the vanes extend between the nozzle ring and the        nozzle portion; and    -   a plurality of spacers connected between the nozzle portion of        the insert and the nozzle ring for securing the nozzle ring to        the insert and maintaining an axial spacing between the nozzle        portion of the insert and the nozzle ring.

In accordance with some embodiments of the invention, the turbinehousing includes a surface that directly contacts a surface of thenozzle ring that axially faces toward the insert for axially locatingthe nozzle ring relative to the turbine housing and for sealing theinterface between the nozzle ring and turbine housing. The nozzle ringis structured and arranged relative to the center housing such thatradial locating of the nozzle ring is performed by the center housing,or by a member arranged intermediate the nozzle ring and center housing.

Thus, the retainer ring of the prior cartridge design is eliminated,which allows the diameter of the turbine housing to be reduced by asignificant amount. This provides a smaller package and enables the costof the turbocharger to be reduced.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a fragmentary sectioned side view of a turbocharger inaccordance with a first embodiment of the invention;

FIG. 2 is an isometric view of the nozzle ring of the first embodiment;

FIG. 3 is a fragmentary sectioned side view of a turbocharger inaccordance with a second embodiment of the invention;

FIG. 4 is an isometric view of the center housing of the secondembodiment;

FIG. 5 is a fragmentary sectioned side view of a turbocharger inaccordance with a third embodiment of the invention;

FIG. 6 is an isometric view of an intermediate sleeve arranged betweenthe nozzle ring and center housing of the third embodiment;

FIG. 7 is a fragmentary sectioned side view of a turbocharger inaccordance with a fourth embodiment of the invention, along line 7-7 inFIG. 8;

FIG. 8 is a cross-sectional view through the nozzle ring and locatingring of the fourth embodiment;

FIG. 9 is a plan view of the locating ring of the fourth embodiment;

FIG. 10 is a fragmentary sectioned side view of a turbocharger inaccordance with a fifth embodiment of the invention;

FIG. 11 is a fragmentary sectioned side view of a turbocharger inaccordance with a sixth embodiment of the invention; and

FIG. 12 is a fragmentary sectioned side view of a turbocharger inaccordance with a seventh embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings in which some but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

A turbocharger 10 in accordance with one embodiment of the invention isillustrated in fragmentary perspective view in FIG. 1. The turbochargercomprises a compressor 12 having a compressor wheel or impeller 14mounted in a compressor housing (not shown) on one end of a rotatableshaft 18. The shaft is supported in bearings 19 mounted in a centerhousing 20 of the turbocharger. The shaft 18 is rotated by a turbinewheel 22 mounted on the other end of the shaft 18 from the compressorwheel, thereby rotatably driving the compressor wheel, which compressesair drawn in through the compressor inlet and delivers the compressedair to the intake of an internal combustion engine (not shown) forboosting the performance of the engine.

The turbocharger also includes a turbine housing 24 that houses theturbine wheel 22. The turbine housing defines a generally annularchamber 26 that surrounds the turbine wheel and that receives exhaustgas from the internal combustion engine for driving the turbine wheel.The exhaust gas is directed from the chamber 26 generally radiallyinwardly through a turbine nozzle 28 to the turbine wheel 22. As theexhaust gas flow through the passages between the blades 30 of theturbine wheel, the gas is expanded to a lower pressure, and the gasdischarged from the wheel exits the turbine housing through a generallyaxial bore 32 therein.

The turbine nozzle 28 is a variable nozzle for varying thecross-sectional flow area through the nozzle so as to regulate flow intothe turbine wheel. The nozzle includes a plurality of vanes 34 that arecircumferentially spaced about the nozzle. Each vane is affixed to a pin36 that passes through an aperture 37 (FIG. 2) in a generally annularnozzle ring 38 that is mounted coaxially with respect to the turbinewheel 22. Each pin 36 is rotatable about its axis within the associatedaperture in the nozzle ring so that the vane can rotate about the axisfor varying the setting angle of the vane. The nozzle ring 38 forms onewall of the flow passage of the nozzle 28. Each of the pins 36 has avane arm 40 affixed to an end of the pin that protrudes out from thenozzle ring 38, and is engaged by a generally annular unison ring 42(also referred to herein as an actuator ring) that is rotatable aboutits axis and that is coaxial with the nozzle ring 38. An actuator (notshown) is connected to the unison ring 42 for rotating it about itsaxis. When the unison ring is rotated, the vane arms 40 are rotated tocause the pins 36 to rotate about their axes, thereby rotating the vanes34 so as to adjust the vane setting angles and thereby vary thecross-sectional flow area through the nozzle 28.

With reference to FIG. 2, the nozzle ring 38 has a radially inwardlyfacing surface 44 that engages a radially outwardly facing surface 21(FIG. 1) of the center housing 20 to radially locate the nozzle ringrelative to the center housing. The contact between the nozzle ring andcenter housing can be over less than a full circumference of the centerhousing, in order to reduce thermal transfer between the nozzle ring andcenter housing. For example, as shown in FIG. 2, the radially inwardlyfacing surface 44 of the nozzle ring can include a plurality ofcircumferentially spaced recesses 46 such that contact between thenozzle ring and center housing occurs only at the surfaces 48 betweenthe recesses 46. Thus, in this embodiment, the nozzle ring 38 isdirectly centered on the center housing 20, which facilitates accurateconcentricity between the nozzle ring and center housing.

With reference again to FIG. 1, the variable vane mechanism is providedin the form of a cartridge 50 that is installable into and removablefrom the turbocharger as a unit. The cartridge 50 comprises the nozzlering 38, vanes 34, pins 36, vane arms 40, and unison ring 42. Thecartridge further comprises an insert 52 that has a tubular portion 54sealingly received into the bore 32 of the turbine housing, and a nozzleportion 56 extending generally radially out from one end of the tubularportion 54, the nozzle portion 56 being axially spaced from the nozzlering 38 such that the vanes 34 extend between the nozzle ring 38 and thenozzle portion 56. The radially outer surface of the tubular portion 54has at least one circumferential groove, and preferably has two axiallyspaced grooves as shown in FIG. 1, in each of which a sealing ring (notshown) is retained for sealingly engaging the inner surface of the bore32. Advantageously, the outer diameter of the tubular portion 54 of theinsert is slightly less than the inner diameter of the bore 32 so that aslight gap is defined therebetween, and only the sealing rings 58 makecontact with the inner surface of the bore 32. Additionally, there is agap between the nozzle portion 58 and the adjacent end of the turbinehousing at the end of the bore 32. In this manner, the insert 52 ismechanically and thermally decoupled from the turbine housing 24.

A plurality of spacers (not shown) are connected between the nozzleportion 56 of the insert 52 and the nozzle ring 38 for securing thenozzle ring to the insert and maintaining the desired axial spacingbetween the nozzle portion of the insert and the nozzle ring. Eachspacer passes through an aperture in the nozzle portion 56 and has anenlarged head on the side of the nozzle portion 56 that faces away fromthe nozzle 28. Each spacer also has a pair of enlarged shoulders axiallyspaced along the length of the spacer such that one shoulder abuts theopposite side of the nozzle portion 56 and the other shoulder abuts thefacing surface of the nozzle ring 38, thereby setting the axial spacingbetween the nozzle ring and nozzle portion. An end portion of eachspacer passes through an aperture in the nozzle ring 38 and the distalend of this end portion is upset to form an enlarged head to capture thenozzle ring. Advantageously, the spacers are formed of a material havinggood high-temperature mechanical properties and a relatively low thermalconductivity, such as stainless steel (e.g., grade 310 stainless steel)or the like, so that the nozzle ring 38 and insert 52 are effectivelythermally decoupled from each other.

The turbine housing 24 has an annular radially inwardly extendingprojection 70 that engages the surface of the nozzle ring 38 facingaxially toward the insert 52. The engagement between the projection 70and the nozzle ring 38 preferably is along a full 360° circumference ofthe nozzle ring so as to substantially seal the interface between theturbine housing and the nozzle ring. The projection 70 also assists thespacers in restraining the nozzle ring with respect to axial movement inthe direction toward the insert 52. Advantageously, the turbine housing24 has a radially inner surface 72 facing toward a radially outersurface 74 of the nozzle ring 38, and the turbine housing surface 72 isgreater in diameter than the nozzle ring surface 74 such that there is agap 76 between these surfaces. The gap 76 accommodates radialdisplacement of the nozzle ring relative to the turbine housing, such asmay occur through differential thermal growth or other causes. Thisprovides an effective stress decoupling of the variable vane cartridge50 from the turbine housing.

The cartridge 50 can include a heat shroud 80 that is captively retainedbetween the nozzle ring 38 and the center housing 20 when the cartridgeis installed onto the center housing. The heat shroud 80 providessealing between the nozzle ring and center housing to prevent hotexhaust gas from migrating between these parts into the cavity in whichthe vane arms and unison ring 42 are disposed. The heat shroud 80 cancomprise a generally annular member formed of a resiliently elasticmaterial such as spring steel or the like, and the shroud can beconfigured so that it is compressed in the axial direction between thenozzle ring 38 and the center housing 20 so that the restoring force ofthe shroud urges the shroud firmly against surfaces of the nozzle ringand center housing to substantially seal against these surfaces. Inparticular, as shown in FIG. 1, the heat shroud 80 has a radially outerportion that engages a surface of the nozzle ring that faces axiallytoward the center housing, and a radially inner portion that engages asurface of the center housing that faces axially toward the nozzle ring.These surfaces of the nozzle ring and center housing compress andelastically deform the heat shroud 80 between them.

The limited contact area between the nozzle ring 38 and center housing20 is provided in the above-described first embodiment by configuringthe nozzle ring with the recesses 46 so that only the surfaces 48between the recesses contact the center housing's radially outer surface21. Alternatively, in a second embodiment illustrated in FIGS. 3 and 4,such limited contact area can be provided by configuring the centerhousing 120 to include a plurality of circumferentially spacedprojections 122 that project radially outwardly to contact the radiallyinwardly facing surface 144 of the nozzle ring 138 and provide radialcentering of the nozzle ring relative to the center housing. As in thefirst embodiment, a heat shroud 80 is elastically deformed bycompression between oppositely facing surfaces of the nozzle ring andcenter housing.

The above-described first and second embodiments feature direct contactbetween the nozzle ring and center housing for centering of the nozzlering. Alternatively, in a third embodiment depicted in FIGS. 5 and 6, anintermediate ring or sleeve 260 is disposed between the nozzle ring 238and the center housing 220. More particularly, the sleeve 260 has aradially outer surface on which a plurality of circumferentially spaced,radially outwardly extending projections 262 are formed. The radiallyinner surface 264 of the sleeve can be a continuous cylindrical surfaceas shown, or alternatively could include similar projections extendingradially inwardly. The projections 262 contact the radially inwardlyfacing surface 244 of the nozzle ring 238, while the radially innersurface 264 of the sleeve contacts the radially outer surface 221 of thecenter housing 220. The sleeve is radially centered on the centerhousing, and the nozzle ring in turn is radially centered on the sleeve.Thus, the nozzle ring is radially centered on the center housing by wayof the intermediate sleeve. An elastically deformed heat shroud 80 isincluded as in the prior embodiments.

The sleeve 260 is illustrated as a continuous 360° ring in FIG. 6.Alternatively, however, the sleeve can be a split ring that iselastically deformable, such as a compensating ring or the like. Thesplit ring can be configured so that in its relaxed state it is largerin diameter than the inner surface of the nozzle ring; the split ring isthen compressed radially inwardly to reduce its diameter so that thenozzle ring can be installed about the split ring.

A fourth embodiment of the invention is illustrated in FIGS. 7-9. Anintermediate locator ring 360 is disposed between the nozzle ring 338and the center housing 320 for radially centering the nozzle ringrelative to the center housing, similar to the third embodiment. Unlikethe sleeve of the third embodiment, however, the locator ring 360 alsoorients the nozzle ring in the circumferential direction relative to thecenter housing. The locator ring 360 comprises a 360° ring. The radiallyouter surface of the locator ring defines a plurality ofcircumferentially spaced projections 362 that project radially outwardlyand collectively define a surface that contacts the radially inwardlyfacing surface 344 of the nozzle ring 338 for radially centering thenozzle ring. The radially inner surface 364 of the locator ring definesa plurality of circumferentially spaced recesses 366 for creating airgaps between the locator ring and center housing. These air gaps,together with the air gaps created between the radially outwardprojections 362 on the outer surface of the locator ring, help to reduceheat conduction from the nozzle ring through the locator ring to thecenter housing.

The inner surface 364 of the locator ring 360 also defines twodiametrically opposite flats 368 that engage corresponding flats formedon the radially outer surface of the center housing 320 such that thelocator ring 364 is located in a predetermined circumferentialorientation relative to the center housing.

The radially outer surface of the locator ring further defines aradially outwardly extending protuberance 370 at one circumferentiallocation thereof. The protuberance 370 extends out to a larger radiusthan the circumferentially spaced projections 362. The radially innersurface of the nozzle ring 338 at one circumferential location thereofdefines a recess 372 for receiving the protuberance 370 such that thenozzle ring is oriented in a predetermined circumferential orientationwith respect to the locator ring. Accordingly, the nozzle ring islocated in a predetermined circumferential orientation relative to thecenter housing, via the locator ring.

A fifth embodiment of the invention is shown in FIG. 10. A spring sleeve460 is disposed between the radially inwardly facing surface 444 of thenozzle ring 438 and the radially outer surface 421 of the center housing420. The spring sleeve comprises a 360° ring of an elasticallydeformable material such as spring steel or the like. The spring sleevehas a generally S-shaped cross-section, and thus has resilience in theradial direction when compressed radially and also has resilience in theaxial direction when compressed axially. The center housing 420 definesa surface 422 that axially faces toward the nozzle ring 438 and extendsradially out from and forms a corner with the radially outer surface 421of the center housing. The nozzle ring 438 defines an opposite surface445 that axially faces toward the center housing and extends radially infrom and forms a corner with the radially inwardly facing surface 444. Aheat shroud 480 is disposed between the nozzle ring and center housingand its radially outer periphery is sandwiched between a radially outerportion of the spring sleeve 460 and the axially facing surface 445 ofthe nozzle ring. A radially inner portion of the spring sleeve 460engages the axially facing surface 422. The spring sleeve 460 isradially compressed between the radially facing surfaces 444, 421 of thenozzle ring and center housing respectively, and is axially compressedbetween the axially facing surfaces 445, 422 of the nozzle ring andcenter housing respectively. The spring sleeve thus provides radialcentering of the nozzle ring relative to the center housing.

The outer periphery of the heat shroud 480 is axially compressed orcaptured between the spring sleeve and nozzle ring. However, unlikeprior embodiments, the inner periphery of the heat shroud does notengage the center housing and thus the heat shroud is not elasticallydeformed. Accordingly, the heat shroud 480 need not be an elasticallydeformable structure, but can be substantially inelastic. The heatshroud 480 is referred to herein as an “unloaded” heat shroud, whereasthe heat shroud 80 of prior embodiments is referred to as a “loaded”heat shroud.

A sixth embodiment of the invention is depicted in FIG. 11. In thisembodiment, the centering of the nozzle ring 538 relative to the centerhousing 520 is performed by an elastically deformable, loaded heatshroud 580. The nozzle ring defines a radially inwardly facing surface544 and a surface 545 that axially faces the center housing and forms acorner with the radially facing surface 544. The outer periphery of theheat shroud 580 is engaged in this corner. The center housing similarlydefines a radially outwardly facing surface 521 and a surface 522 thataxially faces the nozzle ring and forms a corner with the radiallyfacing surface 521. The inner periphery of the heat shroud 580 isengaged in this corner. The heat shroud is configured such that it isslightly radially compressed between the radially facing surfaces 544,521 of the nozzle ring and center housing respectively, and is axiallycompressed and elastically deformed between the axially facing surfaces545, 522 of the nozzle ring and center housing respectively. The heatshroud thus radially centers the nozzle ring relative to the centerhousing, and also provides sealing between these parts.

A seventh embodiment of the invention is illustrated in FIG. 12. In thisembodiment, the center housing 620 includes an axially extending annularflange 625 that passes radially outwardly of the radially outer surface639 of the nozzle ring 638. There is a radial gap between the flange 625and the outer surface 639 of the nozzle ring. The flange defines aradially inner surface in which a circumferential recess or groove 626is formed. A resiliently deformable seal ring 670 is disposed in thegroove 626. The inner diameter of the seal ring in its relaxed state issmaller than the inner diameter of the flange 625 and slightly smallerthan the outer diameter of the surface 639 of the nozzle ring. Thenozzle ring 638 is inserted into the opening of the center housing 620delimited by the flange 625 and this causes the seal ring 670 to beradially compressed between the radially outer surface 639 of the nozzlering and the radially inner surface of the groove 626 in the flange 625.In this manner, the seal ring centers the nozzle ring relative to thecenter housing.

The seal ring 670 can comprise a metal ring that has a hollow circularcross-section as shown. Alternatively, the ring can comprise othermaterials and/or can have other cross-sectional shapes. The seal ring670 not only provides radial centering of the nozzle ring but alsoprovides sealing between the nozzle ring and center housing at the outerdiameter of the nozzle ring. Sealing at the inner diameter of the nozzlering can be provided by a loaded heat shroud 680 similar to previouslydescribed embodiments.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A turbocharger having a variable-nozzle turbine, comprising: aturbine assembly comprising a turbine housing and a turbine wheelmounted in the turbine housing and connected to a rotatable shaft forrotation therewith, the turbine housing defining a chamber surroundingthe turbine wheel for receiving exhaust gas from an engine and forsupplying the exhaust gas to the turbine wheel, the turbine assemblydefining a nozzle leading from the chamber generally radially inwardlyto the turbine wheel; a compressor assembly comprising a compressorhousing and a compressor wheel mounted in the compressor housing andconnected to the rotatable shaft for rotation therewith; a centerhousing connected between the compressor housing and the turbinehousing; a generally annular nozzle ring and an array of vanescircumferentially spaced about the nozzle ring and disposed in thenozzle such that exhaust gas flows between the vanes to the turbinewheel, the vanes being rotatably mounted to the nozzle ring such thatthe vanes are rotatably adjustable for regulating exhaust gas flow tothe turbine wheel, wherein the center housing includes an axiallyextending annular flange that passes radially outwardly of a radiallyouter surface of the nozzle ring, the flange defining a radially innersurface; and a seal member disposed between and contacting the radiallyouter surface of the nozzle ring and the radially inner surface of theflange so as to radially center the nozzle ring relative to the centerhousing.
 2. The turbocharger of claim 1, wherein the seal membercomprises a resiliently deformable seal ring.
 3. The turbocharger ofclaim 2, wherein the seal ring is resiliently deformed in the radialdirection between the radially outer surface of the nozzle ring and theradially inner surface of the flange.
 4. A turbocharger having avariable-nozzle turbine, comprising: a turbine assembly comprising aturbine housing and a turbine wheel mounted in the turbine housing andconnected to a rotatable shaft for rotation therewith, the turbinehousing defining a chamber surrounding the turbine wheel for receivingexhaust gas from an engine and for supplying the exhaust gas to theturbine wheel, the turbine assembly defining a nozzle leading from thechamber generally radially inwardly to the turbine wheel; a compressorassembly comprising a compressor housing and a compressor wheel mountedin the compressor housing and connected to the rotatable shaft forrotation therewith; a center housing connected between the compressorhousing and the turbine housing, the center housing having a radiallyoutwardly facing surface and an axially facing surface that faces towardthe turbine assembly; a generally annular nozzle ring and an array ofvanes circumferentially spaced about the nozzle ring and disposed in thenozzle such that exhaust gas flows between the vanes to the turbinewheel, the vanes being rotatably mounted to the nozzle ring such thatthe vanes are rotatably adjustable for regulating exhaust gas flow tothe turbine wheel, wherein the nozzle ring includes a radially inwardlyfacing surface; a spring sleeve having resilience in the radialdirection when compressed radially and resilience in the axial directionwhen compressed axially, the spring sleeve being disposed between theradially inwardly facing surface of the nozzle ring and the radiallyoutwardly facing surface of the center housing, the spring sleeve beingradially compressed between the respective radially facing surfaces ofthe nozzle ring and the center housing so as to radially center thenozzle ring relative to the center housing.
 5. The turbocharger of claim4, further comprising a heat shroud having a radially outer portiondisposed between and contacting the spring sleeve and an axially facingsurface of the nozzle ring.
 6. The turbocharger of claim 5, wherein thespring sleeve is elastically deformed between the heat shroud and theaxially facing surface of the center housing.
 7. The turbocharger ofclaim 6, wherein the heat shroud is free from contact with the centerhousing.